Long-lived and temperature-independent emission from a novel ruthenium(II) complex having an arylborane charge-transfer unit

Article abstract of DOI:10.1021/ic1020669

We report the redox, absorption, and emission characteristics of the tris(1,10-phenanthroline)ruthenium(II) complexes [Ru(phen)₃] ²⁺ bearing a (dimesityl)boryldurylethynyl (DBDE) charge-transfer (CT) unit at the 4 (4BRu²⁺)

Full text of DOI:10.1021/ic1020669

Inorg. Chem. 2011, 50, 1603–1613 1603
DOI: 10.1021/ic1020669
Long-Lived and Temperature-Independent Emission from a Novel Ruthenium(II)
Complex Having an Arylborane Charge-Transfer Unit
,
†
†
†
,†,‡
Eri Sakuda,* Yuki Ando, Akitaka Ito, and Noboru Kitamura*
†
‡
Department of Chemistry, Faculty of Science, Department of Chemical Sciences and Engineering,
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan
Received October 12, 2010
We report the redox, absorption, and emission characteristics of the tris(1,10-phenanthroline)ruthenium(II) complexes
2
þ 2þ
Ru(phen) ] bearing a (dimesityl)boryldurylethynyl (DBDE) charge-transfer (CT) unit at the 4 (4BRu ) or 5
3
[
2
þ
2
þ
(
maximum wavelengths at 473 and 681 nm, respectively, which were shifted to longer wavelengths by 25 and 74 nm,
5BRu ) position of one of the three phen ligands. In acetonitrile at 298 K, 4BRu showed absorption and emission
2þ
respectively, compared with the relevant value of 5BRu , 448 and 607 nm, respectively. The effects of a fluoride ion
on the absorption and emission spectra of the complexes demonstrated that the CT interaction between the π-electron
system in the phen ligand (π(aryl)) and the vacant p orbital on the boron atom (p(B)) in the DBDE group (i.e.,
π(aryl)-p(B) CT) participated in the excited states of the complexes in addition to the Ru(II)-to-phen metal-to-ligand
2
þ
2þ
CT (MLCT) interaction. Reflecting such synergistic MLCT/π(aryl)-p(B) CT, both 4BRu and 5BRu exhibited
2
intense emission at 298 K with a quantum yield of 0.11. Furthermore, the emission lifetime of 4BRu was as long as
þ
1
2 μs and almost independent of the temperature (T = 280-330 K). The present study indicated that the nonemissive
2
þ
dd excited triplet state did not participate to nonradiative decay in the MLCT excited triplet state of 4BRu . The effects
of the synergistic MLCT/π(aryl)-p(B) CT interactions on the redox, absorption/emission, and photophysical
characteristics of 4BRu and 5BRu are discussed in detail.
2
þ
2þ
Introduction
preferable. Nonetheless, such a complex with a low-energy
metal-to-ligand charge transfer (MLCT) absorption band
A variety of polypyridine ruthenium(II) (Ru(II)) complexes
have been hitherto designed/synthesized, and their photo-
chemical and photophysical properties were studied exten-
sively in the past decades aiming at applications of the
complexes to photosensitizers/photocatalysts in solar energy
3
and, thus, a low-energy MLCT excited triplet state ( MLCT*
em
in the energy of ν ) exhibits, in general, a short excited-state
em
3
lifetime (τ ) because the coupling between the MLCT* and
ground-state potential surfaces for nonradiative decay in-
3
1
creases with a lowering of the MLCT* energy, as predicted
conversion systems. One of the main issues of such studies is
from the energy-gap dependence of the nonradiative decay
the development of a novel Ru(II) complex having a high
light absorption efficiency (i.e., large molar absorption
coefficient) in the visible region and a long excited-state
lifetime to achieve efficient conversion of solar energy to
chemical energy. For efficient utilization of solar radiation, a
complex having a low-energy absorption transition(s) is
em 2
rate constant (k ): ln k µ ν . Furthermore, the excited-
nr
nr
state lifetime of an ordinary Ru(II) complex represented by
RuL3 [L = 2,2 -bipyridine (bpy) or 1,10-phenanthroline
phen)] shows a large temperature (T) dependence, and T
2þ
0
(
em
elevation, in general, gives rise to a large decrease in τ
3
owing to thermal activation from MLCT* to the none-
3
mitting dd excited triplet state ( dd*) and subsequent fast
*
To whom correspondence should be addressed. E-mail: sakueri@
sci.hokudai.ac.jp; kitamura@sci.hokudai.ac.jp.
1) (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S.
3
3,4
nonradiative decay from dd* to the ground state.
(
Although it has been reported that some Ru(II) complexes
em
Chem. Rev. 1996, 96, 759. (b) Polo, A. S.; Itokazu, M. K.; Iha, N. Y. M. Coord.
Chem. Rev. 2004, 248, 1343. (c) Alstrum-Acevedo, J. H.; Brennaman, M. K.;
Meyer, T. L. Inorg. Chem. 2005, 44, 6802. (d) Huynh, M. H.; Dattelbaum, D. M.;
Meyer, T. J. Coord. Chem. Rev. 2005, 249, 457. (e) Rau, S.; Sch €a fer, B.; Gleich,
D.; Anders, E.; Rudolph, M.; Friedrich, M.; G €o rls, H.; Henry, W.; Vos, J. G.
Angew. Chem., Int. Ed. 2006, 45, 6215. (f) Nazeeruddin, M. K.; Klein, C.; Liska,
P.; Gr €a tzel, M. Coord. Chem. Rev. 2005, 249, 1460. (g) Liu, F.; Concecion, J. J.;
Jurss, J. W.; Cardolaccia, T.; Templeton, J. L.; Meyer, T. J. Inorg. Chem. 2008, 47,
show T-independent τ probably due to an increase in the
3
3
4
MLCT*- dd* energy gap of the complex, this sometimes
accompanies the low-energy MLCT excited state of the
(2) (a) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. (b)
Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (c) Allen, G. H.; White,
R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. Soc. 1984, 106, 2613. (d) Kober,
E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 3722.
(e) Chen, P.; Meyer, T. J. Chem. Rev. 1998, 98, 1439.
1
727. (h) Concepcion, J. J.; Jurss, J. W.; Brennaman, M. K.; Hoertz, P. G.;
Patrocinio, A. O. T.; Murakami Iha, N. Y.; Templeton, J. L.; Meyer, T. J. Acc.
Chem. Res. 2009, 42, 1954.
r 2011 American Chemical Society
Published on Web 01/31/2011
pubs.acs.org/IC

1
604 Inorganic Chemistry, Vol. 50, No. 5, 2011
em
Sakuda et al.
complex and, therefore, a short τ , as predicted from the
energy-gap dependence of k . These discussions suggest that
the low-energy MLCT absorption/emission and the long
excited-state lifetime of a Ru(II) complex are mutually
contradicting issues. One exceptional case is τ of Ru(II)
characterized by the presence of the vacant p orbital on the
boron atom (p(B)), and the CT interaction between the π
orbital of the aryl group (π(aryl)) and p(B) [π(aryl)-p(B) CT]
gives rise to characteristic spectroscopic and photophysical
properties of the derivative as a represented example, being
nr
em
8
having an aromatic hydrocarbon substituent(s) (ArH =
those of tri-9-anthrylborane and other derivatives. By in-
pyrene, anthracene, and so forth) on the ligand, in which
troduction of a triarylborane substituent to the ligand of a
transition-metal complex, one may expect synergistic inter-
actions between the π(aryl)-p(B) CT in the triarylborane
unit and MLCT in the transition-metal complex, realizing
novel spectroscopic and photophysical functions, not ob-
tained by the triarylborane or metal complex alone. In
practice, we succeeded in the synthetic tuning of the emission
3
the thermal equilibrium between MLCT* and the ππ*
em
excited triplet state of ArH considerably elongates τ of
the complex compared with the relevant complex without
5
em
2þ
ArH: as an example, τ = 150 μs for RuL
(L =
3
5f
4
-pyrenylphen) in CH CN at 300 K. In the simple
MLCT*- dd* nonradiative decay regime, the τ value of
a Ru(II) complex is limited to less than 7 μs in solution at
3
3
3
em
em
0
0
00
quantum yield (Φ ) and lifetime of a 2,2 :6 ,2 -terpyri-
6
0
0
00
room temperature.
dineplatinum(II) (tpy = 2,2 :6 ,2 -terpyridine) complex by
0
For synthetic modulation of the spectroscopic/photophy-
introduction of a (dimesityl)phenylborane group at the 4
position of tpy, B-tpy: [Pt(B-tpy)Cl] , Φ = 0.011, and τ
0.6 μs in CHCl at room temperature. Because it has been
known that [Pt(tpy)Cl] in solution at room temperature is
þ
em
em
sical properties of a transition-metal complex, we proposedin
=
7
7
2
006 the use of a triarylborane-appended ligand. The
3
þ
electronic structure of a triarylborane derivative is best
9
nonluminescent, introduction of the triarylborane CT unit
remarkably influences the photophysical properties of the
(
3) (a) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853. (b)
þ
Van Houten, J.; Watts, R. J. Inorg. Chem. 1978, 17, 3381. (c) Durham, B.; Caspar,
J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4803. (d)
Barrigelletti, F.; Juris, A.; Balzani, V.; Belser, P.; von Zelewsky, A. J. Phys. Chem.
[Pt(B-tpy)Cl] complex. After our first report on [Pt(B-
þ
10
tpy)Cl] , several research groups reported transition-metal
11
12
13
10a,b
(Pt(II), Ir(III), Ru(II), Re(I), or Cu(I)
) complexes
1
987, 91, 1095. (e) Juris, A.; Balzani, V.; Barrigelletti, F.; Campagna, S.; Belser,
bearing an arylborane unit(s) on the ligand, and some of the
complexes are shown to be luminescent in solution at room
P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (f) Ferrando, S. R. L.;
Maheroot, U. S. M.; Deshayes, K. D.; Kinstle, T. H.; Ogawa, M. Y. J. Am. Chem.
Soc. 1996, 118, 5783. (g) Macatangay, A.; Zheng, G. Y.; Rillema, D. P.; Jackman,
D. C.; Merkert, J. W. Inorg. Chem. 1996, 35, 6823. (h) Zheng, G. Y.; Rillema,
D. P. Inorg. Chem. 1996, 35, 7118. (i) Hammarstr €o m, L.; Alsins, J.; B €o rje, A.;
Norrby, T.; Zhang, L.; Åkermark, B. J. Photochem. Photobiol. A: Chem. 1997,
10b-d,11,13
temperature.
Nonetheless, detailed experiments on
the photophysical properties of the complexes bearing an
arylborane unit(s) have not been reported yet: T dependences
em
1
02, 139. (j) Hammarstr €o m, L.; Barigelletti, F.; Flamigni, L.; Indelli, M. T.;
of the emission spectrum and τ , radiative/nonradiative rate
Armaroli, N.; Calogero, G.; Guardigli, M.; Sour, A.; Collin, J.-P.; Sauvage, J.-P. J.
Phys. Chem. A 1997, 101, 9061. (k) Wu, F.; Riesgo, E.; Paualora, A.; Kipp, R. A.;
Schmehl, R. H.; Thummel, R. P. Inorg. Chem. 1999, 38, 5620. (l) Benniston,
A. C.; Chapman, G.; Harriman, A.; Mehrabi, M.; Sams, C. A. Inorg. Chem. 2004,
constants, and so forth. We anticipate that syn-
ergistic MLCT/π(aryl)-p(B) CT interactions can tune the
spectroscopic and photophysical properties of various transi-
tion-metal complexes.
4
3, 4227. (m) Thompson, D. W.; Fleming, C. N.; Myron, B. D.; Meyer, T. J. J.
Phys. Chem. B 2007, 111, 6930. (n) Abrahamsson, M.; Becker, H.-C.;
Hammarstr €o m, L.; Bonnefous, C.; Chamchoumis, C.; Thummel, R. P. Inorg.
Chem. 2007, 46, 10354.
In this Article, we report here that the Ru(II) complex
bearing a (dimesityl)boryldurylethynyl (DBDE) group at the
2þ 2þ
4
(4BRu ) or 5 (5BRu ) position of a 1,10-phenanthroline
(
4) (a) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am.
ligand shows quite intriguing spectroscopic and photophysi-
cal properties: see Chart 1 for the structures. In particular, we
Chem. Soc. 1984, 106, 2613. (b) Henderson, L. J., Jr.; Cherry, W. R. J.
Photochem. 1985, 28, 143. (c) Wacholtz, W. F.; Auerbach, R. A.; Schmehl,
R. H. Inorg. Chem. 1986, 25, 227. (d) Kawanishi, Y.; Kitamura, N.; Tazuke, S.
Inorg. Chem. 1989, 28, 2968. (e) Wang, Y.; Peres, W.; Zheng, G. Y.; Rillema,
D. P. Inorg. Chem. 1998, 37, 2051. (f) Rillema, D. P.; Blanton, C. B.; Shaver, R. J.;
Jackman, D. C.; Boldaji, M.; Bundy, S.; Worl, L. A.; Meyer, T. J. Inorg. Chem.
2þ
found that 4BRu exhibited essentially a low-energy and
em
em
T-independent emission in solution with Φ = 0.11 and τ
1
=
em
2þ
2 μs. To the best of our knowledge, the τ value of 4BRu
1
992, 31, 1600. (g) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene,
is the longest among those of the polypyridine ruthenium(II)
F. R.; Meyer, T. J. Inorg. Chem. 1996, 35, 2242. (h) Zheng, G. Y.; Wang, Y.;
Rillema, D. P. Inorg. Chem. 1996, 35, 7118. (i) Wang, Y.; Greg, W. P.; Zheng,
G. Y.; Rillema, D. P. Inorg. Chem. 1998, 37, 2051.
complexes hitherto reported. The synthesis and redox, spec-
2þ
troscopic, and photophysical properties of 4BRu
and
(
5) (a) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.;
Piotrowiak, P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119,
1012. (b) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel, R. Chem. Commun.
999, 735. (c) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955.
(8) (a) Kitamura, N.; Sakuda, E. J. Phys. Chem. A 2005, 109, 7429.
1
1
(b) Kitamura, N.; Sakuda, E.; Yoshizawa, T.; Iimori, T.; Ohta, N. J. Phys. Chem.
A 2005, 109, 7435. (c) Sakuda, E.; Tsuge, K.; Sasaki, Y.; Kitamura, N. J. Phys.
Chem. B 2005, 109, 22326. (d) Kitamura, N.; Sakuda, E.; Iwahashi, Y.; Tsuge, K.;
Sasaki, Y.; Ishizaka, S. J. Photochem. Photobiol. A: Chem. 2009, 207, 102.
(d) Michalec, J. F.; Behune, S. A.; McMillin, D. R. Inorg. Chem. 2000, 39, 2708.
(e) Tyson, D. S.; Bialecki, J.; Castellano, F. N. Chem. Commun. 2000, 2355.
(f) Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J. Phys. Chem. A
(
e) Kitamura, N.; Sakuda, E.; Ando, E. Chem. Lett. 2009, 38, 938. (f) Sakuda, E.;
Ando, Y.; Ito, A.; Kitamura, N. J. Phys. Chem. A 2010, 114, 9144.
9) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994, 33,
22.
(10) (a) Sun, Y.; Ross, N.; Zhao, S. B.; Huszarik, K.; Jia, W.-L.; Wang,
2
001, 105, 8154. (g) Morales, A. F.; Accorsi, G.; Armaroli, N.; Barigelletti, F.;
Pope, S. J. A.; Ward, M. Inorg. Chem. 2002, 41, 6711. (h) Maubert, B.;
MaClenagham, N. D.; Indelli, M. T.; Campagna, S. J. Phys. Chem. A 2003,
(
7
1
07, 447. (i) Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.;
Castellano, F. N. Inorg. Chem. 2003, 42, 1394. (j) Benniston, A. C.; Harriman,
A.; Lawwrie, D. J.; Mayeux, A. Phys. Chem. Chem. Phys. 2004, 6, 51. (k) Wang,
X.-Y.; Del Guerzo, A.; Schmehl, R. H. J. Photochem. Photobiol. C: Rev. 2004,
R.-Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510. (b) Zhao,
S.-B.; M Cormick, T.; Wang, S. Inorg. Chem. 2007, 46, 10965. (c) Sun, Y.; Wang,
c
S. Inorg. Chem. 2009, 48, 3755. (d) Hudson, Z. M.; Zhao, S.-B.; Wang, R.-Y.;
Wang, S. Chem.;Eur. J. 2009, 15, 6131. (e) Rao, Y.-L.; Wang, S. Inorg. Chem.
2009, 48, 7698. (f) Sun, Y.; Wang, S. Inorg. Chem. 2010, 49, 4394.
(11) (a) Zhou, G.; Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.;
Lin, Z.; Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499. (b) You,
Y.; park, S. Y. Adv. Mater. 2008, 20, 3820. (c) Zhao, Q.; Li, F.; Liu, S.; Yu, M.;
Liu, Z.; Yi, T.; Huang, C. Inorg. Chem. 2008, 47, 9256.
5
, 55. (l) Kozlov, D. V.; Tyson, D. S.; Goze, C.; Ziessel, R.; Castellano, F. N.
Inorg. Chem. 2004, 43, 6083–6092. (m) Balazs, G. C.; del Guerzo, A.; Schmehl,
R. H. Photochem. Photobiol. Sci. 2005, 4, 89. (n) Gu, J.; Chen, J.; Schmehl, R. H.
J. Am. Chem. Soc. 2010, 132, 7338.
(
6) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; Taylor & Francis: Boca Raton, FL, 2006.
7) Sakuda, E.; Funahashi, A.; Kitamura, N. Inorg. Chem. 2006, 45,
0670.
(
(12) Wade, C. R.; Gabbaı
(13) Lam, S.-T.; Zhu, N.; Yam, V. W.-W. Inorg. Chem. 2009, 48, 9664.
¨
, F. P. Inorg. Chem. 2010, 49, 714.
1

Article
Chart 1. Structures of 4BRu and 5BRu
Inorganic Chemistry, Vol. 50, No. 5, 2011 1605
2þ
2þ
2þ
Synthesis of 4BRu PF
6
Salt. After an oven-dried Schlenk
(4-Br-
(78 mg, 0.078 mmol), CuI (1.8 mg, 0.0094 mmol),
and Pd(PPh ) Cl (3.3 mg, 0.0046 mmol) were added, and the
tube was evacuated and filled with an Ar gas, [Ru(phen)
phen)](PF
2
6 2
)
3
2
2
tube was evacuated and filled with an Ar gas. An Ar-gas-purged
CH CN (2.2 mL)/triethylamine (1 mL) mixture was added to the
tube and stirred for 15 min at room temperature. A tetrahydrofuran
THF; 1.6 mL) solution of EDDB (40 mg, 0.098 mmol) was then
added dropwise to the reaction mixture. The mixture was stirred
gas atmosphere and then cooled to
3
(
at 50 °C for 2.5 h under a N
2
ambient temperature. The insoluble solids were removed by
filtration through Celite, and the concentrated filtrate was
3
added dropwise to a sufficient amount of a CH CN/n-hexane
mixture, giving red precipitates. The crude product was purified
by column chromatography (LH-20, ethanol/CH CN = 1/1
3
(
v/v)). The product was dissolved in a CH
v/v)), and then diethyl ether was added to the mixture. The
3 2
CN/H O mixture (1/1
(
organic layer was separated, washed with water, and evaporated
2
under reduced pressure, affording a PF salt of 4BRu as red
þ
6
1
3
solids (51 mg, 50%). H NMR (270 MHz, CD CN): δ 1.95 (s,
1
7
0
8
2H), 2.04 (s, 6H), 2.52 (s, 6H), 6.81 (s, 4H), 7.61-7.68 (m, 5H),
2
þ
5
BRu are reported in detail, and the origin of the long-lived
.73 (d, 1H, J = 5.6 Hz), 8.02-8.08 (m, 5H), 8.12 (dd, 1H, J =
.92 and 5.2 Hz), 8.26 (s, 4H), 8.35 (d, 1H, J = 9.6 Hz),
2þ
and T-independent emission lifetime of 4BRu is discussed.
6 2 2
.59-8.65 (m, 6H). Anal. Calcd for C66H57BF12N P Ru H O:
3
Experimental Section
þ
C, 58.54; H, 4.39; N, 6.21. Found: C, 58.68; H, 4.49; N, 6.05.
).
2þ
2
Synthesis of 4BRu and 5BRu . The synthetic routes to
2þ
ESI-MS: m/z 523 ([M - 2PF
]
6
2
þ
2þ
2þ
Synthesis of [Ru(phen) {5-(dimesitylboryldurylethynyl)-
2
4
BRu
and 5BRu
are shown in Chart 2. 4BRu was
2þ
2þ
.
phen}] PF
6
Salt: 5BRu
Synthesis of 5-Ethynyl-1,10-phenan-
synthesized by the Sonogashira-Hagiwara cross-coupling re-
action between (ethynylduryl)dimesitylborane (EDDB) and a
1
8
throline (5-E-phen). . 5-Bromo-1,10-phenanthroline (1.0 g,
1
9
2þ
3.9 mmol) prepared by the reported method was dissolved in
50 mL of THF, and the solution was purged with an Ar gas
stream for 10 min at room temperature. Into the mixture were
added (trimethylsilyl)acetylene (0.85 mL, 6.0 mmol), diiso-
propylamine (5 mL, 35 mmol), Pd(PPh ) Cl (250 mg, 0.36
PF salt of [Ru(phen) (4-Br-phen)] . EDDB was synthesized
6 2
1
according to the literature, and [Ru(phen) (4-Br-phen)] was
4
2þ
2
prepared by the reaction between cis-dichlorobis(1,10-phenan-
1
5
16,17
throline)ruthenium(II) (cis-Ru(phen)
2
Cl
þ
was synthesized by reacting [Ru(phen) (5-ethynyl-
2
) and 4-Br-phen.
2
3
2
2
5
BRu
2
mmol), and CuI (83 mg, 0.44 mmol), and the mixture under an
Ar gas atmosphere was heated at reflux temperature for 7 h
under stirring. After cooling to room temperature, insoluble
solids were filtered off and the filtrate was evaporated
to dryness. The residues were dispersed in an aqueous KCN
(200 mg, 20 mL)/CH OH (50 mL) mixture, and the suspen-
3
sion was treated by ultrasonic irradiation for 1 h. The solution
was extracted with CHCl (50 mL) three times, and the com-
3
phen)](PF ) with (iododuryl)dimesitylborane (IDDB) in the
2
6
presence of dichlorobis(triphenylphosphine)palladium(II) [Pd-
(
PPh
3 2 2
) Cl ] and CuI.
2
All of the chemicals used for the synthesis of 4BRu and
þ
2
BRu , purchased from Wako Pure Chemical Ind., Kanto
þ
5
Chemical Co. Inc., Tokyo Kasei Kogyo Co. Ltd., or Sigma-
Aldrich Co., were used as supplied. Column chromatography
was carried out by using Merck silica gel 60 (particle size
bined CHCl
dryness. The crude product was purified successively by flash
column chromatography (SiO , CH Cl /CH OH = 95/5 (v/v))
and recrystallization from a CH Cl /n-hexane mixture, yielding
3 2 4
solution dried over Na SO was evaporated to
0
.063-0.200 mm) or GE Healthcare Sephadex LH-20.
H NMR spectra were recorded on a JEOL JME-EX270 FT-
1
2
2
2
3
NMR system (270 MHz). The chemical shifts of the spectra
determined in CDCl3 or CD CN were given in ppm, with
2
2
3
5
-E-phen as off-white solids (0.39 g, 49%).
tetramethylsilane being an internal standard (0.00 ppm). Elec-
trospray ionization mass spectometry (ESI-MS) spectra were
recorded on a Waters micromass ZQ spectrometer.
2þ
Synthesis of [Ru(phen) (5-E-phen)] PF Salt. An ethanol
2 6
solution (100 mL) of 5-E-phen (100 mg, 0.49 mmol) and cis-
Ru(phen) Cl (250 mg, 0.47 mmol) under an Ar gas atmosphere
2
2
Synthesis of [Ru(phen) {4-(dimesitylboryldurylethynyl)-
2
2
þ
2þ
was heated at reflux temperature for 7 h under stirring. After
cooling to room temperature, the solvent was evaporated to
dryness. The residues were dissolved in a minimum amount of
water, and to the solution was added dropwise a concentrated
phen}]2 PF6 Salt: 4BRu
.
Salt. A suspension of cis-Ru(phen)
Synthesis of [Ru(phen) (4-Br-
2
þ
phen)] PF
6
2
Cl
2
(60 mg,
15
16,17
0.11 mmol) and 4-Br-phen
glycol (8 mL) was purged with an N gas stream for 15 min.
(37 mg, 0.14 mmol) in ethylene
2
4 6
aqueous NH PF solution. The precipitated solids were col-
Upon microwave irradiation (200 W), the reaction mixture
became a homogeneous solution. After microwave irradiation
for 2 min, the reaction mixture was allowed to cool to ambient
temperature and a saturated aqueous KPF solution was added
6
dropwise to the solution, giving red precipitates. The precipitates
lected by suction filtration and washed successively with water
and an acetone/diethyl ether mixture. After drying in vacuum,
the crude product was purified successively by flash column
chromatography (Al
from an acetone/ethanol/n-hexane mixture, yielding [Ru-
2 3 3
O and CH CN) and recrystallization
were collected by suction filtration, affording [Ru(phen)
(
2
-
1
(
phen)
2
(5-E-phen)](PF
6
)
2
as red solids (0.27 g, 58%).
H
6 2
4-Br-phen)](PF ) as red solids (100 mg, 88%). ESI-MS: m/z
2
þ
NMR (270 MHz, CD CN): δ 4.13 (s, 1H), 7.65 (m, 7H), 8.04
3
3
61 ([M - 2PF6] ).
(m, 6H), 8.25 (s, 4H), 8.55 (m, 5H), 8.85 (d, 2H, J = 8.3 Hz).
Anal. Calcd for C38 12Ru: C, 47.76; H, 2.53; N, 8.79.
Found: C, 47.76; H, 2.84; N, 8.54.
24 6 2
H N P F
(
(
14) Yamaguchi, S.; Shirasaka, T.; Tamao, K. Org. Lett. 2000, 2, 4129.
15) Sprintschnik, G.; Sprintschnik, H. W.; Kirsch, P. P.; Whitten, D. G.
J. Am. Chem. Soc. 1977, 99, 4947.
(
16) Poe, D. P.; Eppen, A. D.; Whoolery, S. P. Talanta 1980, 27, 368.
(18) Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org. Chem. 1996, 61, 6535.
(19) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447.
(
17) Bijeire, L.; Legentil, L.; Bastide, J.; Darro, F.; Rochart, C.; Delfourne, E.
Eur. J. Org. Chem. 2004, 1981.

1
606 Inorganic Chemistry, Vol. 50, No. 5, 2011
Sakuda et al.
2
Chart 2. Synthetic Routes to 4BRu and 5BRu
þ
2þ
(a) (1) (Trimethylsilyl)acetylene, Pd(PPh
200 W), 2 min, and then KPF . (c) Pd(PPh
reflux, 18 h; (2) KCN, MeOH, sonication, 1 h. (e) cis-[Ru(phen)
THF, 70 °C, 15 h.
3
)
2
Cl
Cl
2
, CuI, Et
2
NH, reflux, 18 h; (2) KOH, MeOH/THF, rt, 12 h. (b) cis-[Ru(phen)
, CH
Cl
2
Cl
2
], ethylene glycol, MW
)
(
6
3
)
2
2
, CuI, NEt
3
3
CN/THF, 50 °C, 2.5 h. (d) (1) (Trimethylsilyl)acetylene, Pd(PPh
3
2
Cl
2
, CuI, Et
2
NH,
2
2
], EtOH, reflux, 7 h, and then KPF . (f) IDDB, Pd(PPh Cl , CuI, NEt
6
3
)
2
2
3
, CH
3
CN/
2
þ
2þ
Synthesis of 5BRu PF
tube was evacuated and filled with an Ar gas, IDDB (180 mg,
.36 mmol), CuI (6.4 mg, 0.034 mmol), and Pd(PPh Cl (13
6
Salt. After an oven-dried Schlenk
dryness under reduced pressure, affording a PF
6
salt of 5BRu
as orange solids (113 mg, 28%). H NMR (270 MHz, CD CN):
1
3
0
3
)
2
2
δ 1.95 (s, 12H), 2.06 (s, 6H), 2.25 (s, 6H), 6.81 (s, 4H), 7.59-7.67
(m, 5H), 7.71 (dd, 1H, J = 5.3 and 8.4 Hz), 8.00-8.08 (m, 6H),
8.25 (s, 4H), 8.55 (t, 2H, J = 4.2 Hz), 8.60 (dd, 4H, J = 1.2 and
2.5 Hz), 8.95 (d, 1H, J = 1.1 and 8.4 Hz). Anal. Calcd for
C H BF N P Ru H O: C, 58.16; H, 4.44; N, 6.16. Found: C,
mg, 0.018 mmol) were added, and the tube was evacuated and
filled with an Ar gas. An Ar-gas-purged THF (4 mL)/triethyla-
mine (3 mL) solution was added to the tube and stirred at room
temperature for 15 min. After CH CN (ca. 1 mL) was added to
3
the mixture, a CH CN (5.5 mL) solution of [Ru(phen) (5-E-
2
66 57
12
6
2
3
2
2þ
58.14; H, 4.39; N, 5.98. ESI-MS: m/z 523 ([M - 2PF ] ).
3
6
phen)](PF
6
)
2
(290 mg, 0.30 mmol) was added dropwise, and the
gas
Spectroscopic and Electrochemical Measurements. Solvents
for spectroscopic and electrochemical measurements were dis-
resultant solution was heated at 70 °C for 15 h under a N
2
2
tilled prior to the use. Absorption and corrected emission
0
atmosphere. After the reaction, the mixture was filtered through
Celite and the filtrate was evaporated to dryness under reduced
pressure. The residues were dissolved in a minimum amount of
acetone, and the product was reprecipitated in n-hexane. The
precipitates collected by suction filtration were purified by
2þ
2þ
2þ
spectra of 4BRu , 5BRu , and Ru(phen)3 were measured by
using a Hitachi UV-3300 spectrophotometer and a Hamamatsu
multichannel photodetector (PMA-11, excitation wavelength =
355 nm), respectively. The absolute emission quantum yields
em
column chromatograpy (LH-20, ethanol/CH
After evaporation of the eluted solution under reduced pressure,
the product was dissolved in a CH CN/H O mixture (1/1 (v/v)),
3
CN = 1/1 (v/v)).
(Φ ) of the complexes were measured by a Hamamatsu C9920-02
3
2
and then diethyl ether was added to the mixture. The organic
layer was separated, washed with water, and evaporated to
(20) Perrin, D. D.: Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pargamon Press: New York, 1980.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1607
system equipped with an integrating sphere and a red-sensitive
multichannel photodetector (PMA-12, excitation wavelength =
2
1
4
<
50 nm). The absorbance of a sample solution was set at
0.05 at the excitation wavelength. Emission lifetime measure-
ments were conducted by using a streak camera (Hamamatsu
Photonics, C4334) as a photodetector at 355 nm excitation
(
LOTIS TII Ltd., 355 nm). A liquid N2 cryostat (DN1704
optical Dewar and 3120 temperature controller, Oxford In-
struments) was used to control the sample temperature. For
emission spectroscopy, sample solutions were deaerated by
purging an Ar gas stream over 30 min.
Cyclic voltammetry was conducted by using an electrochem-
ical analyzer (BAS, ALS-701A). The concentrations of the
Ru(II) complexes in N,N-dimethylformamide (DMF) were
-4
-3
set at 5 ꢀ 10 mol dm (=M), and 0.1 M tetra-n-butylammo-
nium hexafluorophosphate (TBAPF ) was used as a supporting
6
electrolyte. Sample solutions were deaerated by purging an Ar
gas stream over 20 min prior to the experiments. A three-
electrode system was employed by using Pt working, Pt auxiliary,
and saturated calomel electrode (SCE) reference electrodes.
Theoretical Calculations. The calculations on the electron
densities in the highest occupied molecular orbital (HOMO)/
32þ
2þ
2þ
Figure 1. CVs ofRu(phen) (black), 4BRu (red), and 5BRu (blue)
in DMF in the presence of 0.10 M TBAPF . Scan rate = 100 mV s
-
1
.
6
2þ
2þ 2þ 2þ
, 4BRu , and 5BRu in DMF (0.1 M
lowest unoccupied molecular orbital (LUMO) levels of 4BRu
and 5BRu were conducted on the Gaussian 09W programs.
Table 1. Redox Potentials of Ru(phen)
3
a
2þ
22
6
TBAPF )
Optimizations of the structures of the complexes were per-
formed by using the B3LYP density functional theory (DFT).
The LANL2DZ and 6-31* basis sets were used to treat the
ruthenium atom and all other atoms, respectively. Time-depen-
dent DFT (TD-DFT) calculations were then performed to
estimate the energies and oscillator strengths of the 10 lowest-
energy transitions. The contours of the electron density were
plotted by using Chem3D 11.0.
potential/V (vs SCE)
ox
red(1)
red(2)
red(3)
E
red(4)
E
E
E
E
2
þ
Ru(phen)
3
þ1.26
þ1.28
þ1.30
þ1.28
þ1.30
-1.27
-1.09
-1.20
-1.21
-1.25
-1.42
-1.41
-1.43
-1.39
-1.40
-1.74
-1.75
-1.67
-1.68
-1.71
2þ
4
4
5
5
BRu
BRu þ F
-2.12
-1.89
2þ
2þ
2þ
- b
BRu
BRu þ F
-b
a
2þ
-3
2þ
b
-4
2þ
[
Ru(phen) ] =1.1 ꢀ 10 M, [4BRu ] = 4.9ꢀ 10 M, [5BRu ] =
Results and Discussion
-4
-
-4
5
.4 ꢀ 10 M, and [F ] = 5.5 ꢀ 10 M. Data were compiled from Figure
2
Redox Potentials of 4BRu and 5BRu . The DBDE
þ
2þ
S1 in the Supporting Information.
2
þ
group at the 4 or 5 position of the phen ligand in 4BRu
red(1,2,3)
23
and E(Ru ), respectively). The E and E
2þ/þ
þ/0
red(1,2,3)
2
þ
of the three phen ligands (E
; E(Ru
), E(Ru ),
or 5BRu , respectively, will act as an electron-accepting
unit owing to the presence of p(B), and this will reflect on
the redox potentials of the complexes. The cyclic voltam-
0
/-
ox
2
þ
values of Ru(phen)3 determined in the present study
(þ1.26, -1.27, -1.42, and -1.74 V in DMF, respectively)
corresponded very well to the reported values (þ1.35,
2
þ
2þ
mograms (CVs) thus observed for 4BRu and 5BRu in
DMF are shown in Figure 1 together with that of Ru-
2
3b
2
phen)3 as a reference complex, and the redox poten-
þ
-1.36, -1.46, and -1.80 V in CH CN, respectively).
3
(
tials of the complexes (vs SCE) are summarized in Table 1.
2þ
2þ
4
BRu and 5BRu also exhibited one metal oxidation
2
þ
wave and three consecutive reduction waves similar to
As seen in Figure 1, the CV of Ru(phen)
exhibited a
)) and three reduc-
tion waves responsible for consecutive one-electron reductions
3
2þ 2þ
those of Ru(phen) . The E values of 4BRu and
3
ox
ox
3þ/2þ
metal oxidation wave (E ; E(Ru
2
þ
2þ
5
BRu were almost identical with that of Ru(phen)3 ,
with the value of both complexes being þ1.28 V, indicat-
ox
ing that the introduction of the DBDE group to phen
resulted in a minor effect on the E value.
(
21) (a) Suzuki, K.; Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara,
T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S. Phys. Chem. Chem. Phys. 2009,
1, 9850. (b) Ishida, H.; Tobita, S.; Hasegawa, Y.; Katoh, R.; Nozaki, K. Coord.
Chem. Rev. 2010, 254, 2449.
22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
ox
In marked contrast to E , the presence of the DBDE
1
red(1,2,3)
group on the phen ligand largely influenced E
of
(
2þ
2þ
2þ
red(1)
both 4BRu and 5BRu . In the case of 4BRu , E
was observed at a more positive potential (-1.09 V) by 180
M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson,
G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.;
Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro,
F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.;
Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.;
Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.;
Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts,
R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth,
G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas,
O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
2
þ
mV compared with that of Ru(phen)
(-1.27 V), while
3
red(2,3)
E
with the relevant value of Ru(phen)
(-1.41 and -1.75 V, respectively) agreed very well
2þ
within (10 mV.
3
red(1)
2þ
The E value of 5BRu (-1.21 V) was shifted in the
2
positivedirection by60mVrelativetothatofRu(phen)3
þ
,
while the potential shift by an introduction of the DBDE
group to the phen ligand in 5BRu was rather moderate
2þ
2þ
red(2,3)
compared with that of 4BRu . The E
5BRu were also shifted to the positive potential com-
pared with those of 4BRu or Ru(phen)3 . For both
4BRu and 5BRu , furthermore, the fourth reduction
values of
2þ
2
þ
2þ
(
23) (a) Kalyanasundaram, K. Photochemistry of Polypyridine and Por-
2þ 2þ
red(4)
phyrin Complexes; Academic Press: London, 1992. (b) Bouskila, A.; Drahi, B.;
Amouyal, E.; Sasaki, I.; Gaudemer, A. J. Photochem. Photobiol. A: Chem. 2004,
waves (E ) were observed at -2.12 and -1.89 V,
2
respectively, which was not observed for Ru(phen)3
þ
.
1
63, 381.

1
608 Inorganic Chemistry, Vol. 50, No. 5, 2011
Sakuda et al.
To assign the fourth reduction waves observed for
BRu and 5BRu , we studied the effects of a fluoride
2
þ 2þ
4
ion (tetra-n-butylammonium fluoride, TBAF) on the
redox potentials of the complexes, as shown by the CVs
in Figure S1 in the Supporting Information, and the redox
-
potentials of the complexes in the presence of F ([TBAF] =
5
-
4
.5 ꢀ 10 M) are included in Table 1. It has been
reported that triarylboranes and transition-metal com-
plexes bearing a triarylborane unit(s) act as fluoride ion
7
,10,11b,11c,12,24
-
sensors
because a F ion is likely to coor-
dinate with p(B), and this results in large changes in the
redox and absorption/emission properties of the com-
pounds compared with those in the absence of F . It is
-
-
worth noting that a PF6 ion used as a supporting
electrolyte in the CV experiments (i.e., TBAPF ) or a
2
þ
2þ
Figure 2. Absorption spectra of Ru(phen)
3
(black), 4BRu (red),
and 5BRu (blue) in acetonitrile at 298 K. The perpendicular bars shown
6
2þ
2
counter anion of 4BRu or 5BRu cannot bind with
þ
2þ
-
by the red and blue colors represent the oscillator strengths of several
absorption transitions in 4BRu and 5BRu , respectively. See also
Figure 4 and Tables S1 and S2 in the Supporting Information.
p(B) because a PF6 ion is too large to access p(B) on the
boron atom surrounded by the bulky dimesityl and duryl
2þ
2þ
2
groups in 4BRu or 5BRu . Figure S1 in the Supporting
þ
2þ
Information and Table 1 clearly demonstrate the disap-
wave and the negative potential
4
45 nm have been assigned to the ligand-centered (LC)
red(4)
2
5
pearance of the E
shift of the E
and MLCT transitions, respectively. Owing to the close
similarities of the absorption spectral band shapes and
maximum wavelengths of the two main bands of 4BRu
red(1)
2þ
2þ
value of 4BRu (-1.20 V) or 5BRu in
-
2
þ
the presence of F (-1.25 V) compared with that in the
-
abs
2þ
absence of F : -1.09 or -1.21 V, respectively. These
(
λ
= 264 and 473 nm) or 5BRu (263 and 448 nm) with
red(1)
red(4)
2
þ
results indicate that E
influence of p(B). Because the E
and Ered(4) are under the strong
those of Ru(phen)3 , the shorter- and longer-wavelength
absorption bands observed for the complexes could be
tentatively assigned to the LC (ππ* transition in phen)
and MLCT transitions, respectively. The MLCT absorp-
wave is not observed
in Ru(phen)3 , the reduction wave could be assigned to
2
þ
2
that of the DBDE group in 4BRu or 5BRu : electron
þ
2þ
2
þ
occupation by p(B). Furthermore, the negative potential
tion band of 4BRu was observed at the lower energy
red(1)
2þ 2þ
observed for 4BRu or 5BRu in the
abs
3
-1
-1
shift of E
presence of F relative to that in the absence of F
[
wavenumber (ν ) = 21.14 ꢀ 10 cm ] by 1330 cm
þ
3
-
-
2
abs
3
compared with that of Ru(phen)
(ν = 22.47 ꢀ 10
red(1)
-
1
demonstrates that E is essentially responsible for
the reduction of the phen ligand having the DBDE group
cm ). Because the HOMO/LUMO energy gap of
2
þ
ox
red(1)
4
BRu estimated from the E and E
values in
) is smaller than that of
Ru(phen)3 by 1290 cm and the value agrees very well
2
in 4BRu or 5BRu . The rather insensitive nature of
þ
2þ
ox
red(1)
Table 1 (ΔE = E - E
red(2,3)
E
2þ 2þ
-
2
þ
-1
abs
of 4BRu and 5BRu to F indicates that the
2þ
reduction waves of the complexes are ascribed to the
reduction of the phen ligands without DBDE. It is worth
emphasizing that the DBDE group introduced to the 4
position of phen shows a stronger electron-accepting ability
compared with that to the 5 position of phen. The reduction
with the difference in ν
between 4BRu and Ru-
phen)3 , the lower-energy shift of the MLCT absorp-
2
þ
(
tion band of 4BRu relative to that of Ru(phen)3 will
2
þ
2þ
be reasonably explained by the electron-accepting ability
of the DBDE group in 4BRu . Similarly, the small
lower-energy shift of the MLCT band observed for
2
þ
2
þ
2þ
potentials observed for 4BRu or 5BRu , different from
2þ
2
þ
abs
abs
3
-1
-1
those of Ru(phen)3 , should reflect the spectroscopic and
photophysical properties of the complexes, and the results
are described in the following sections.
Absorption Spectra of 4BRu and 5BRu . Figure 2
shows the absorption spectra of the three Ru(II) com-
5BRu (ν = 22.32 ꢀ 10 cm ) by 150 cm compared
2þ
with ν of Ru(phen)
will also be accounted for by the
3
small ΔE value of the complex relative to that of Ru-
2
þ
2þ
2
þ
-1
(
phen)3 : 320 cm
Figure 2 also indicates that both 4BRu and 5BRu
exhibit a new absorption band in 300-400 nm, not
.
2
þ
2þ
plexes in CH CN at 298 K, and the spectroscopic data are
3
2
þ
summarized in Table 2: the absorption maximum wave-
observed for Ru(phen) . It is worth emphasizing that
3
abs
2
þ
2þ
length (λ ) and the molar absorption coefficient (ε). The
the absorption band similar to that of 4BRu or 5BRu
at 300-400 nm can be observed for Ru(phen) (4-
2
þ
abs
absorption bands of Ru(phen)3
at λ
= 263 and
2
2
þ
2þ
durylethynylphen) or Ru(phen) (5-durylethynylphen)
(
2
2
i.e., a 4BRu or 5BRu type complex without a dime-
þ 2þ
(
24) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001,
1
2
23, 11372. (b) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Organomet. Chem.
002, 652, 3. (c) Kubo, Y.; Yamamoto, M.; Ikeda, M.; Takeuchi, M.; Shinkaia, S.;
sitylboryl group) but not for the ligand itself, as reported
in Figure S2 in the Supporting Information. Therefore,
the absorption band observed for 4BRu or 5BRu at
Yamaguchi, S.; Tamao, K. Angew. Chem., Int. Ed. 2003, 42, 2036. (d) Melami,
M.; Gabbaï, F. P. J. Am. Chem. Soc. 2005, 127, 9680. (e) Parab, K.;
Venkatasubbaiah, K.; J €a kle, F. J. Am. Chem. Soc. 2006, 128, 12879. (f) Zhou,
Z.; Xiao, S.; Xu, J.; Liu, Z.; Shi, M.; Li, F.; Yi, T.; Huang, C. Org. Lett. 2006, 8,
2þ
2þ
300-400 nm is responsible for the Ru(II)-to-π(duryl-
ethynylphen) CT transition. Nonetheless, one should
3
911. (g) Zhou, Z.; Yang, H.; Shi, M.; Xiao, S.; Li, F.; Yi, T.; Huang, C.
ChemPhysChem 2007, 8, 1289. (h) Hudnall, T. W.; Kim, Y.-M.; Bebbington,
M. W.; Bourissou, D.; Gabbaï, F. P. J. Am. Chem. Soc. 2008, 130, 10890. (i) Kim,
Y.; Gabbaï, F. P. J. Am. Chem. Soc. 2009, 131, 3363. (j) Hudson, Z. M.; Wang, S.
Acc. Chem. Res. 2009, 42, 1584. (k) Hudnall, T. W.; Chiu, C.-W.; Gabbaï, F. P.
Acc. Chem. Res. 2009, 42, 388. (l) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge,
S.; Gabbaï, F. P. Chem. Rev. 2010, 110, 3958.
(25) (a) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (b)
Ferguson, J.; Herren, F.; Krausz, E. R.; Vrbancich, J. Coord. Chem. Rev. 1985,
64, 21. (c) Juris, A.; Barigelletti, F.; Campagna, S.; Balzani, V.; Belser, P.;
von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (d) Wallace, A. W.; Murphy,
W. R., Jr.; Petersen, J. D. Inorg. Chim. Acta 1989, 166, 47.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1609
2þ
2þ
2þ
Table 2. Spectroscopic and Photophysical Properties of Ru(phen)
3
, 4BRu , and 5BRu in Acetonitrile at 298 K
abs
4
-1
-1
em em em
4
/10 s
-1
4
-1
λ
/nm (ε/10 M cm
)
λ
/nm
599
Φ
τ
/μs
0.42
k
r
k
nr/10 s
2
þ
Ru(phen)
3
263 (11), 445 (1.7)
264 (9.5), 376 (3.3), 473 (2.6)
263 (9.1), 373 (2.5), 448 (1.7)
0.045
0.11
0.11
11
0.92
9.2
230
7.4
74
2
2
þ
þ
4
5
BRu
BRu
681
607
12
1.2
2
þ
2þ
Figure 3. Absorption spectra of 4BRu (a) and 5BRu (b) in the
absence and presence of TBAF in acetonitrile at 298 K. TBAF equiv-
2þ
alences were 0, 0.47, 0.93, 1.9, 2.8, and 3.7 for 4BRu and 0, 0.49, 0.99,
2.0, and 3.0 for 5BRu
2þ
.
consider the contribution of the DBDE group [i.e., p(B)]
to the absorption spectrum of 4BRu or 5BRu similar
2
þ
2þ
to that to the redox potentials of the complexes. Therefore,
we studied the effects of a F ion on the absorption
-
spectra of the complexes.
Figure 3 shows the absorption spectral responses of
Figure 4. Electron density maps in the HOMO and LUMO levels of
4BRu and 5BRu whose geometries were optimized by the DFT
calculations at the B3LYP/6-31* level.
2
BRu and 5BRu to a F ion in CH CN: molar
þ
2þ
-
2þ
2þ
4
3
-
concentration ratio of [complex]/[F ] = 1.0/0-3.7 and
.0/0-3.0, respectively. As is clearly seen in the figures,
the absorption band intensities of both complexes at
1
value and the longer-wavelength shift of the MLCT (i.e.,
MLCT/π(aryl)-p(B)) absorption transition. Further-
more, the present results demonstrate that the posi-
tion of the DBDE group introduced to phen is a very
important factor governing the synergistic MLCT/
π(aryl)-p(B) CT interactions and the introduction of
the group to the 4 position of phen results in larger effects
on the electronic structures of the complex compared with
that to the 5 position of phen.
-
3
00-400 nm decrease in the presence of F . This demon-
strates that the dimesitylboryl group or p(B) also con-
tributes to the electronic transition(s) at 300-400 nm and,
thus, the π-electron system in the durylethynyl group
communicates with that of the dimesitylboryl group in
2
þ 2þ
4
BRu or 5BRu , indicating that the absorption band is
ascribed to superposition of the Ru(II)-to-π(duryleth-
ynylphen) CT and Ru(II)-to-π(DBDE)/p(B) CT transi-
tions; we define this synergistic MLCT/π(aryl)-p(B) CT
interaction. Figure 3 indicates, furthermore, that the
presence of p(B) influences the entire electronic absorp-
TD-DFT Calculations of the HOMO/LUMO Levels in
4BRu and 5BRu . To confirm the above discussions,
2
þ
2þ
we conducted TD-DFT calculations, as the electron
density maps at several HOMO/LUMO levels in 4BRu
and 5BRu show in Figure 4. The oscillator strengths ( f )
2
þ
2þ
tion transitions in 4BRu , while the contribution of p(B)
to the MLCT transition of 5BRu is rather small com-
2
þ
2þ
2
pared with that of 4BRu . Thus, the synergistic MLCT/
þ
of the 10 lowest-energy absorption transitions estimated
by the TD-DFT calculations are also included in Figure 2,
and the detailed data are reported in Tables S1 and S2 in
the Supporting Information. According to our calcula-
2
þ
π(aryl)-p(B) CT interactions are stronger in 4BRu
than in 5BRu . In the case of 4BRu , this gives rise to
2
þ
2þ
4
the large ε value of the MLCT transition (2.6 ꢀ 10
-
1
-1
abs
cm at λ = 473 nm) relative to that of 5BRu
2þ
2þ
M
tions, the lowest-energy absorption transition in 4BRu
4
-1
-1
abs
(1.7 ꢀ 10 M cm at λ
indicate that an introduction of the DBDE group to the
phen ligand in 4BRu brings about enhancement of the ε
= 448 nm). The results
is ascribed to the HOMO (48%)/HOMO-1 (52%) f
LUMO transition at 498.61 nm with f = 0.0744 (see
Table S1 in the Supporting Information). The HOMO/
2
þ

1
610 Inorganic Chemistry, Vol. 50, No. 5, 2011
Sakuda et al.
2
þ
HOMO-1 levels of 4BRu are characterized by the
electron densities on the Ru(II) atom and the pyridine
ring in the durylethynylphen system without any contri-
bution of p(B) to the HOMOs, while the excited electron
in the LUMO distributes to both p(B) and the durylethy-
nylphen system. The results clearly support our assign-
2
þ
ment of the lowest-energy absorption band of 4BRu to
the synergistic MLCT/π(aryl)-p(B) CT. Our calcula-
2
þ
tions on the f values of several transitions in 4BRu also
reproduce very well the observed absorption spectrum
seen in Figure 2.
2
þ
In the case of 5BRu , on the other hand, the
HOMO-1 is described by electron localization on the
Ru(II) atom, whereas the electron density in the HOMO
distributes to both p(B) and the durylethynylphen group.
2
þ
The LUMO of 5BRu is characterized by the large
electron densities in the durylethynylphen ligand with a
minor contribution of the density on p(B). The lowest-
32þ
2þ
Figure 5. Emission spectra of Ru(phen)
5BRu (blue) in acetonitrile at 298 K.
(black), 4BRu (red), and
2þ
2
þ
energy absorption transition predicted for 5BRu is the
HOMO-1 f LUMO (47%)/LUMOþ2 (53%) transition
at 473.92 nm with f = 0.0002 without any contribution of
the HOMO level. Because the LUMOþ2 level is localized
on the Ru(II)-phen core, the π(aryl)-p(B) CT character
in the lowest excited state is predicted to be weaker than
show in Figure S3 in the Supporting Information. There-
fore, the emissive MLCT excited states of these complexes
are also under the strong influence of p(B) and, thus, the
synergistic MLCT/π(aryl)-p(B) CT interactions. These
results explicitly demonstrate that an introduction of
the triarylborane CT unit extraordinarily influences the
photoophysical properties of the complexes.
2
þ
that in 4BRu , which agrees very well with the present
observations on both the redox potentials and absorption
Table 2 includes the radiative (k ) and nonradiative rate
r
constants (k ) of the three complexes evaluated by the
nr
2
spectra of 4BRu and 5BRu . Furthermore, our calcu-
þ
2þ
em
em
lations reproduced very well the absorption spectrum of
5
relation Φ = k /(k þ k ) = k τ . The k value of
r
4
r
nr
r
r
2
BRu in Figure 2: see also Table S2 in the Supporting
þ
2þ -1
5BRu (9.2 ꢀ 10 s ) was almost comparable to that of
2
þ
4
-1
2þ
Information. Therefore, the TD-DFT calculations ex-
plain very well the present experimental observations,
and we conclude that the absorption transitions in both
Ru(phen)
(11 ꢀ 10 s ), while that of 4BRu (0.92 ꢀ
3
4
0 s ) was almost one-tenth of the k value of Ru(phen)
-1
2þ
1
Irrespective of the k value, however, the k value is much
.
r
3
r
nr
2
þ
2þ
4
BRu and 5BRu are characterized by synergistic
larger than k for each complex and, thus, the emission
r
lifetimes of these complexes are primarily determined by
knr. As seen in Table 2, the knr values of the complexes
MLCT/π(aryl)-p(B) CT interactions, while the contri-
2
þ
bution of the π(aryl)-p(B) CT is stronger in 4BRu than
2
þ
.
2
þ
in 5BRu
Emission Characteristics of 4BRu
Figure 5 shows the emission spectra of the three Ru(II)
varied dramatically in the sequence of Ru(phen)
(2.3 ꢀ
3
2
þ
2þ
and 5BRu .
6
-1
2þ
5
-1
2þ
1
1
0 s ) > 5BRu (7.4 ꢀ 10 s ) > 4BRu (7.4 ꢀ
4
-1 2þ 2þ
0 s ), and that of 4BRu or 5BRu was ∼31 or
complexes in CH CN at 298 K, where the emission
3
2þ
3.1 times smaller, respectively, than that of Ru(phen)
.
The long emission lifetime of 4BRu or 5BRu is thus
3
intensities of the complexes are normalized to those at
2þ
2þ
em
the maximum wavelengths (λ ). Because the lowest-
energy absorption bands of both 4BRu and 5BRu
essentially responsible for the small knr value.
The k values of Ru(II) or Os(II) complexes have been
frequently discussed on the basis of the energy gap law,
by which the ln knr value correlates linearly with the
2
þ
2þ
nr
are assigned to the MLCT/π(aryl)-p(B) CT transitions,
the emissions from these complexes will also be ascribed to
2
2þ
the MLCT-type emissions. In practice, both 4BRu and
exhibited broad and structureless emissions
em
emission maximum energy (ν ) through the relations
2
þ
5
BRu
em
at λ = 681 and 607 nm, respectively, similar to Ru-
em
2
þ
em
γ0ν
(
of the emission from 4BRu relative to that from 5BRu
phen)3 (λ = 599 nm). The longer wavelength shift
ln knr µ -
ð1Þ
2þ
2þ
pω
2þ
or Ru(phen)3 is a reasonable consequence, as expected
values and the redox potentials of these
where ω is the angular frequency of the vibration(s)
responsible for nonradiative decay and γ is given by
abs
from the λ
complexes.
0
em
em
The Φ
and τ
values evaluated for the Ru(II)
complexes are included in Table 2. Both 4BRu and
em
ν
2
þ
γ0 ¼ ln
- 1
ð2Þ
Spω
2
þ
em
5
BRu showed intense emissions with Φ = 0.11,
whose value was almost 2 times larger than that of Ru-
where S is the parameter related to the vibrational
displacement between the ground- and excited-state po-
2
þ
em
em
(
4
phen) : Φ = 0.045. Furthermore, the τ values of
3
2
þ
2þ
em
BRu aenmd 5BRu were 12 and 1.2 μs, respectively.
tential surfaces. Although the ν term is included in γ0,
this contribution to an energy gap plot has been reported
2þ
Because τ of Ru(phen)
the DBDE group to the 4 or 5 position of the phen ligand
is 0.42 μs, an introduction of
3
to be minor. Therefore, ln k should correlate linearly
nr
em
em
with ν under the assumption of the energy gap law.
nr
2
gives rise to elongation of τ by a factor of ∼29 or 2.9,
2
Nonetheless, it is clear that the k values of 4BRu and
þ
respectively. It is worth noting that the emissions from
both 4BRu and 5BRu are quenched by F , as the data
2
þ
2þ
-
2þ
5BRu
cannot be explained by the energy gap law

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1611
Figure 6. Schematic illustration of the three-state model for the non-
radiative decay of a polypyridine ruthenium(II) complex.
2
þ
em
4
because the lower-energy emission from 4BRu (ν
=
)
3
-1
-1
1
4.7 ꢀ 10 cm ) shows a smaller k value (7.4 ꢀ 10 s
nr
compared with k of the higher-energy emission from
nr
2
þ
em
3
-1
5 -1
5
or Ru(phen)
1
BRu (ν = 16.5 ꢀ 10 cm and k = 7.4 ꢀ 10 s
)
nr
-1
2
þ
em
3
(ν = 16.7 ꢀ 10 cm and k = 2.3 ꢀ
3
nr
6
0 s ). Clearly, one must seek another possible origin of
-1
2
þ
the long emission lifetime or small knr value of 4BRu
other than the energy-gap dependence of knr.
Temperature Dependences of the Emission Lifetimes of
Figure 7. Temperature (T) dependences (280-320 K) of the emission
decay profiles of 4BRu (a) and 5BRu (b) in propylene carbonate.
2þ
2þ
2
þ
2þ
em
4
Ru(II) complex in solution at around room temperature
BRu and 5BRu . Another factor governing τ of a
3
is thermal population of the emitting MLCT* state to the
3
nonemitting dd* state and subsequent fast nonradiative
3
decay from the dd* state to the ground state (S ), as
0
mentioned before: see also Figure 6. Because the
MLCT*- dd* interconversion process requires an acti-
3
3
em
vation energy (ΔE* in Figure 6), τ of Ru(II), in general,
decreases with an increase in the temperature (T). Ac-
3
a,b
cording to Van Houten and Watts
workers,
and Meyer and co-
2
,3c
em
T-dependent τ , τ(T), can be given by eq 3
ꢀ
ꢁꢁ
ΔE
k T
-
1
0
0
0
τðTÞ
¼ ðkr þ knr Þ þ k exp -
ð3Þ
B
0
r
0
where k and k are the T-independent k and k values
nr
r
nr
0
3
of the emissive MLCT* state, respectively, and k and k
B
3
3
are the frequency factor for the MLCT*- dd* inter-
conversion process and the Boltzmann constant, respec-
Figure 8. Temperature (T) dependences of the emission lifetimes of
2
þ 2þ 2þ
(black), 4BRu (red), and 5BRu (blue) in propylene
Ru(phen)
carbonate. The solid curves show the best fits by eq 3.
3
2
tively. In the case of Ru(bpy)3 as an ordinary Ru(II)
þ
em
0
showing a large T-dependent τ , the k and ΔE* values in
1
3
-1
3
3
CH CN have been reported to be 5.8 ꢀ 10
s
and 3800
Table 3. Activation Parameters for the MLCT*- dd* Interconversion Process
3
-
1
2a
em
2þ 2þ 2þ
, 4BRu , and 5BRu in Propylene Carbonate
of Ru(phen)
3
cm , respectively. To reveal the origin of the long τ
value of 4BRu , we studied the T dependence of the
2
þ
ΔE*/cm-1
0
-1
0
0
-1
k /s
k
r
þ knr /s
2þ
emission lifetime of 4BRu , together with those of
2
3
þ
13
5
4
2
þ
2þ
.
Ru(phen)
4BRu
3500
410
2800
5.7 ꢀ 10
2.1 ꢀ 10
2.6 ꢀ 10
1.5 ꢀ 10
6.8 ꢀ 10
9.9 ꢀ 10
5
BRu and Ru(phen)3
Figure 7 shows the emission decay profiles of 4BRu
and 5BRu in propylene carbonate in the T range of
2þ
2þ
4
4
2
þ
11
5
BRu
2
þ
2
þ
2þ
2þ
2
ployed a high boiling point (bp) and an optically trans-
80-330 K. For the T-controlled experiments, we em-
4BRu , 5BRu , and Ru(phen)3 are summarized in
Figure 8. The solid curves shown in Figure 8 are the best
fits of the τ(T) data by eq 3, and the parameters deter-
parent solvent, propylene carbonate (bp = 242 °C),
em
instead of using CH CN (bp = 81.6 °C). The data
mined by the fittings are shown in Table 3. The τ data of
3
2
þ
0
13 -1
demonstrate that the decay profiles of the complexes obey
single-exponential functions irrespective of T. As seen in
Ru(phen)3 could be fitted with k = 5.7 ꢀ 10
s
and
-
1
ΔE = 3500 cm , whose values were almost comparable
em
2þ
2þ
the figure, τ of 5BRu depends strongly on T similar to
to the relevant value of Ru(bpy) mentioned before.
0
The k and ΔE* values of 5BRu were determined to be
2.6 ꢀ 10 s
values determined for 5BRu are somewhat smaller than
3
2
þ
2þ
2þ
that of Ru(phen)
4
or Ru(bpy)3 , whereas that of
3
2
þ
11 -1
-1
BRu is almost independent of T: 10.7 (280 K) ∼ 9.8
and 2800 cm , respectively. Although the
em
μs (330 K). The T dependences of τ determined for
2þ

1
612 Inorganic Chemistry, Vol. 50, No. 5, 2011
Sakuda et al.
2
the relevant value of Ru(phen)3 , it will be concluded
þ
27
and 1.1, or 6.6 and 1.3 μs, respectively. According to
Glazer et al., the origin of the dual emission from the
complex is due to participation of the spatially isolated
3
that the MLCT* state of Ru(phen)
2þ
2þ
or 5BRu under-
goes nonradiative decay through the dd* state similar to
3
3
2
þ 2a
2þ
0
that of Ru(bpy)
τ
.
On the other hand, analysis of the
0
Ru(II)-to-{4-(4 -R-phenylethynyl)phen} and Ru(II)-to-
bpy CT excited states, and the long and short emission
lifetime components of the complex are ascribed to the
former and latter excited states, respectively. They also
reported that the long emission lifetime component of
[Ru(bpy) {4-(phenylethynyl)phen}] is not ascribed to
the emission from the ππ* excited triplet state of the
4-(phenylethynyl)phen ligand itself in the complex. We
also confirmed that no long-lifetime transient responsible
for the ππ* excited triplet state of the ligand in 4BRu
could be observed in nanosecond transient absorption
3
em
data on 4BRu in Figure 8 afforded k and ΔE as
2.1 ꢀ 10 s and 410 cm , respectively. Clearly, these
values are too small to ascribe to the parameters for
thermal activation from MLCT* to the dd* state, and
the ΔE value is rather close to the value corresponding to
the T dependence of the viscosity in propylene carbonate:
6 Therefore, we conclude that the dd* state
does not participate in nonradiative decay in the MLCT*
5
-1
-1
2
8
3
3
2
þ
2
-
1 26
.
3
00 cm
3
2
þ
2þ
state of 4BRu
It is worth emphasizing that the E values of 4BRu
.
ox
2þ
2
þ
0
and 5BRu are the same at þ1.28 V and are comparable
spectroscopy. Thus, the Ru(II)-to-{4-(4 -R-phenyl-
ethynyl)phen} CT excited state of such a complex pos-
2
þ
to that of Ru(phen)
ligand-field-splitting energies of the three complexes are
(þ1.26 V), suggesting that the
3
sesses a very long emission lifetime: 6.6-11.5 μs. It is
3
approximately similar to one another and, thus, the dd*
2þ
worth noting that Ru(phen) {4-(durylethynyl)phen} as
2
2
a reference compound of 4BRu without a dimesitylbor-
þ
energies of the complexes will also be similar, although
this is a very crude approximation. Nevertheless, 4BRu
2
þ
yl group in CH CN at 298 K shows a single-exponential
3
em
em
em
em
em
shows the T-independent τ , while the τ values of
decay with τ = 5.8 μs (λ = 675 nm and Φ = 0.056).
These results indicate that extension of the ligand
π-electron system through the arylacetylene unit at the
4 position of phen can elongate the emission lifetime of the
relevant Ru(II) complex. Nevertheless, the τ value of
Ru(phen) {4-(durylethynyl)phen} is almost half of that
of 4BRu , demonstrating the important role of the
2
BRu and Ru(phen)3 are dependent on T. Because
þ 2þ
5
the emission energy of 4BRu is lower than that of
2
þ
2
þ
2þ
-1
5
BRu or Ru(phen)3 by 1800-2000 cm , one possible
2
explanation for the results on 4BRu in Figure 8 will be
þ
em
3
3
2þ
the large MLCT*- dd* energy gap, which inhibits
thermal activation from MLCT* to dd*. Without non-
2
2þ
3
3
3
radiative decay via the dd* state, therefore, the emission
synergistic MLCT/π(aryl)-p(B) CT in determining the
2
þ
em
2þ
value of 4BRu . Further detailed photophysical
studies on the complexes, including low-temperature
lifetime of 4BRu becomes as long as 12 μs in CH CN at
τ
3
2
98 K. It has been reported that some polypyridine
em
ruthenium(II) complexes show T-independent τ values
similar to that of 4BRu , with the represented examples
being
dazine, 2,2 -bipyrazine, 2,2 -bipyrimidine,
forth as a ligand(s).
because the MLCT* states lie in relatively low energy
compared with that of Ru(phen)3 or Ru(bpy)3 , very
fast nonradiative decay to the ground state occurs as
predicted by the energy-gap dependence of k , and this
experiments, are absolutely necessary to explain the
excited states of both 4BRu and 5BRu , which are
now in progress in our laboratory.
2
þ
2þ
2þ
0
Ru(II)
0
complexes
having
3,3 -bipyri-
4
d
4a
0
4a,f
and so
For these complexes, however,
4
b,c,g-i
Conclusions
3
2þ
In the present study, we demonstrated that 4BRu
showed a low-energy emission (λ = 681 nm and νem
2
þ
2þ
=
em
3
-1
1
4.7 ꢀ 10 cm ) with a long (12 μs in CH CN at 298 K) and
3
nr
2
T-independent emission lifetime (10.7-9.8 μs in 280-330 K)
in solution. Although the low-energy and long-lived emission
properties of a polypyridin eruthenium(II) complex are mu-
tually contradicting issues as predicted by the energy-gap
em
gives rise to short τ values of the complexes. Therefore,
2
BRu showing a long-lived, T-independent, and low-
þ
4
energy emission is quite rare.
Implication to the Excited States of Ru(II) Complexes
Bearing a 4-Arylethynylphen Ligand. As a Ru(II) complex
analogous to 4BRu , Glazer et al. reported the dual-
dependence of k , ithas been shown by the present study that
nr
introduction of a triarylborane CT unit to the 4 position of
2
þ
2þ
2þ
one of the three phen ligands in Ru(phen)3 (i.e., 4BRu )
0
emission behavior of [Ru(bpy) {4-(4 -R-phenylethynyl)-
2
fulfills both requirements through participation of the syner-
gistic MLCT/π(aryl)-p(B) CT interactions in the excited
state. The synergistic CT interactions in 4BRu can also
2
phen}] in CH CN at room temperature, with the long
þ
3
and short emission lifetime components of the complex
with R = -H, -OCH , or -CF being 6.6 and 1.2, 11.5
2þ
3
3
enhance the MLCT-type absorption intensity in the visible
region. Because the complex exhibits a strong redox ability
(
26) There are some arguments on the origin of the small but finite
2þ
2þ
2þ
similar to that of Ru(phen)3 or Ru(bpy)3 , 4BRu with
the long-lived emission will serve as a good photosensitizer/
photocatalyst in solar energy conversion systems.
activation energy for the relatively small T-dependent emission lifetime of
2
0 0
þ 2þ
n
or RuL L 3-n where L and L are polypyridine ligands. These are
0
RuL
1) participation of a fourth MLCT excited state, (2) T dependence of knr
3
(
owing to a T dependence of the emission energy, and (3) nonradiative decay
0
through the higher-energy-lying Ru-L MLCT state other than that from
the lower-energy-lying Ru-L MLCT state. See also refs 4a and 4f in detail.
Alternatively, furthermore, the small ΔE values reported for several Ru(II)
complexes have been discussed in terms of zero-field splitting (ZFS) of the
emissive MLCT excited triplet state (Barigelletti, F.; Belser, P.; von Zelewsky,
A.; Juris, A.; Balzani, V. J. Phys. Chem. 1985, 89, 3680 and ref 3d). In the
present experiments, because the T range studied is rather high (280-330 K)
compared with those reported by Barigelletti et al., the discussion on the role
of ZFS in ΔE is very difficult. Nevertheless, we suppose that the ΔE value
(27) Glazer, E. C.; Magde, D.; Tor, Y. J. Am. Chem. Soc. 2007, 129, 8544.
(28) Glazer et al. also reported the dual-emission behavior of a dinuclear
4þ
Ru(II) complex of [(bpy)
2
Ru{μ-(bis-4-phenanthryl)ethyne}Ru(bpy)
2
]
in
solution at room temperature (Glazer, E. C.; Magde, D.; Torr, Y. J. Am.
Chem. Soc. 2005, 127, 4190). We recently confirmed that the absorption/
emission spectra and dual-emission behavior of the complex mentioned above
could be very well reproduced by our own experiments. Thus, the dual-emission
behavior of the Ru(II) complexes having 4-ethynylphen-type ligands reported by
Glazer et al. will not be due to emissive impurities of the complexes.
-1
2þ
(
410 cm ) observed for 4BRu will be too large to ascribe to the ZFS energy of
the emissive excited state.

Article
Modulation of the absorption and emission characteristics
Inorganic Chemistry, Vol. 50, No. 5, 2011 1613
in a transition-metal complex has thus high potentials to
modulate the spectroscopic and photophysical properties of
the complex, and a study along the lines mentioned above will
open various possibilities of developing potential photosen-
sitizers/photocatalysts in solar energy conversion systems.
2þ
of Ru(phen)3 by introduction of the arylborane CT unit to
þ
phen is not fortuitous because [Pt(B-tpy)Cl] mentioned
em
before also shows intense emission with long τ at room
temperature compared with [Pt(tpy)Cl] without a triaryl-
borane CT unit. Quite recently, furthermore, we reported
þ
7
that a cyclometalated 2-phenylpyridine (ppy) Ir(III) complex
bearing a triarylborane group on ppy, tris[2-{3-(dimesityl-
boryl)phenyl}pyridinato]iridium(III), showed a bright-green
Acknowledgment. E.S. and A.I. acknowledge the Glob-
al COE program of Hokkaido University (Catalysis as
the Basis for Materials Innovation) for a postdoctoral
fellowship (2009) and a research fellowship, respectively.
E.S. also thanks the F3 Project of MEXT, Japan (2010).
em
em
emission with Φ ∼ 1.0 and τ = 1.2 μs in THF, whose
values exceeded the relevant value of Ir(ppy) determined
3
29
under analogous conditions. Several research groups have
also reported intense emission from metal complexes having
Supporting Information Available: Synthesis of EDDB, pur-
ification of other chemicals, details of the TD-DFT calculations,
10a,b,11,13
a triarylborane group(s) as described before.
A
2þ
2þ
combination of MLCT and π(aryl)-p(B) CT interactions
CVs and emission spectra of 4BRu and 5BRu in the absence
and presence of TBAF, and absorption spectra of the reference
complexes without an arylborane CT unit. This material is
available free of charge via the Internet at http://pubs.acs.org.
(
29) Ito, A.; Hirokawa, T.; Sakuda, E.; Kitamura, N. Chem. Lett. 2011,
4
0, 34.